AbstractThe solid‐state structures of (p‐bromoselenobenzoyl)morpholine (2a) and [p‐(dimethylamino)selenobenzoyl]morpholine (2b) were determined by X‐ray diffraction. Both molecules show a flat selenoamide group. The larger contribution of resonance stabilization by the aromatic ring carrying thep‐dimethylamino substituent is reflected by the smaller interplanar angle Θ between the aromatic ring and the selenoamide group [53.3(1)° vs. 81.1(1)°] and by the shorter length of the CSe bond [1.824(5) Å vs. 1.840(3) Å]. The Gibbs free energy of activation of C‐N bond rotation (ΔG≠rot) of fivep‐substituted (selenobenzoyl)morpholines was determined by dynamic13C NMR. The activation barriers were found to range from 61.6 kJ/mol (X = NNMe2) to 75.1 kJ/mol (X = H). The ΔG≠rotvalues of the corresponding (thiobenzoyl)morpholines were found to be from 3.2 kJ/mol (X = NMe2) to 5.0 kJ/mol (X = H) lower. In both cases, ΔG≠rotshowed an excellent linear